1. Field of the Invention
The invention relates to a device for measuring temperatures without mechanical contact, comprising a bolometer arranged as a Wheatstone bridge, a power supply connected to a diagonal arm of the bridge, and a detector for measuring an unbalance voltage in another diagonal arm of the bridge. The bolometer comprises elements sensitive to temperature variations, some of the elements being shielded and some of the elements being not shielded so as to allow or not to allow, respectively, the temperatures of the elements to be changed by the net thermal radiation between said elements and an object arranged opposite the bolometer, and further comprises an element or arrangement for producing additional heating of the elements.
The device can be used for temperature measurements without mechanical contact in a variety of uses.
A bolometer is a thermal detector which absorbs radiation and transforms this into heat, which causes the temperature of the bolometer to rise. A property of the bolometer, generally its electrical resistance, varies with the temperature, which enables the intensity of the radiant flux incident on the bolometer to be measured. Thus, when the radiation emanates from a hot body placed in front of the bolometer, it is possible to determine the temperature of the hot body.
2. Description of the Prior Art
A common bolometer configuration is that of a Wheatstone bridge including a thermistor, i.e. a temperature-sensitive component, intended to receive the radiation, and three other components, which are shielded from the radiation. The three other components are used as references in order to compensate for temperature variations to which the sensing element is subjected. Generally, the bridge is balanced in the absence of radiation, the two bridge arms which are energized in parallel having equal resistances. The optimum sensitivity is obtained when the components forming the bridge have electrical resistances equal to or close to one another.
A temperature measurement device based on this principle is described in the document DE 26 07 806. It includes a bolometer comprising a single thermistor and 3 fixed resistors which are insensitive to temperature variations. The device has a high detection sensitivity in combination with a low power consumption and it does not require any thermostatic device to adjust the bolometer in the absence of radiation. For automatic adjustment of the bridge when it is put into operation, the thermistor is heated by varying the current through the bridge, in accordance with said document. On the basis of measurements of the unbalance voltage of the bridge, a feedback circuit causes the bridge current to be varied so as to balance and thus adjust the bolometer and make it ready for use. Subsequently, the radiation flux is varied at a given rate so as to generate an unbalance signal at the same rate.
For processing of the measurement signal, correction circuits enable the unbalance signal to be corrected by making allowance for the current through the bridge which causes a rise in the thermistor temperature due to the adjustment. However, such a temperature measuring device provides measurements which are still inaccurate because they do not allow for the emissivity factors which distinguish the bolometer from the body emitting the radiation.